In recent years, networks such as a WPAN (Wireless Personal Area Network) and a sensor network, which include compact and low-power-consumption wireless terminals, are attracting attention. As a system similar to these networks, there is a system using active RF tags, each tag actively emitting a wireless signal. A wireless terminal used in these networks has a low data rate (from several kbps to several hundreds of kbps), and a short wireless-signal reaching distance (from several meters to several tens of meters). However, the wireless terminal is compact, and extremely low in power consumption so that it is drivable for several years with a battery embedded therein. Further, there is a passive type wireless terminal needing no battery.
These wireless networks are expected to be applied to a system in which compact wireless terminals are attached to individual articles, and when a large number of the articles enter a communication area of a control terminal as a base station, identification, tracking, and management for these articles are performed by wireless communication.
Further, the wireless networks are also expected to be applied to a system in which a control terminal as a base station is set on a school gate or the like, students commuting to the school are given compact wireless terminals, and authentication of each terminal is conducted when each student passes through the school gate, thereby to detect that the student has passed through the gate.
As one of conventional wireless communication systems to be used in the WPAN, there has been a system using a slot CSMA (Carrier Sense Multiple Access) method (e.g., see Patent Literature 1).
(a) of FIG. 17 shows a network diagram of a wireless communication system disclosed in Patent Literature 1. The wireless communication system includes a base station 2000 that manages a network and connects the same to a backborn network, and terminals 1000-1 to 1000-3 that communicate with the base station 2000 via the wireless network. These terminals 1000-1 to 1000-3 perform communication with sharing the same channel CH1.
(b) of FIG. 17 shows an example of the slot CSMA method disclosed in Patent Literature 1. In a time slot 1, the terminal 1000-1 performs carrier sense after variable-length random backoff to confirm that the wireless network is not occupied, and transmits data to the base station 2000. The base station 2000 returns an ACK to the terminal after a fixed-length ACK (Acknowledgment) return interval has elapsed. Returning the ACK from the base station 2000 allows the terminal 1000-1 to recognize that data reception has succeeded.
In a time slot 2, as in the time slot 1, the terminal 1000-2 transmits data to the base station 2000, and the base station returns an ACK to the terminal after a fixed-length ACK return interval has elapsed.
As described above, in the conventional art, the terminals 1000-1 to 1000-3 perform carrier sense after random backoff, from the beginnings of the respective time slots, and then transmit data. Thereby, collisions of data are reduced, and the quality of communication is improved.
Meanwhile, as another conventional wireless communication system, there is a system performing communication using a plurality of frequency channels (e.g., see Patent Literature 2). In the communication system using a plurality of frequencies which is disclosed in Patent Literature 2, the communication channel qualities are determined according to the communication link conditions, e.g., the RSSI (Received Signal Strength Indicator, received signal power), the bit error rate, and the packet error rate, and communication is performed using high-quality channels.